1. Field of the Invention
The present invention relates generally to patch antennas, array antennas, and mounting boards having the same, and more particularly to a patch antenna and an array antenna used for GPS (Global Positioning System) and ETC (Electronic Toll Collection System), and a mounting board having the same.
2. Description of the Related Art
In general, a patch antenna, which is a planar antenna, has a rectangular or circular shape. FIG. 1 is a perspective view of a patch antenna 110 of an MSL feeding type. As shown in FIG. 1, in the case of feeding to an antenna pattern by microstrip line (MSL) feeding, a matching circuit 106 that performs impedance matching is provided between an MSL 102 and an antenna part 101 since the MSL 102 has an impedance (Z0=50 Ω) different from that of the input end of the antenna part 101. The matching circuit 106 is a circuit of a specific frequency (λ/4, λ=wavelength) whose impedance is the square root of the product of the impedance of the input end of the antenna part 101 and the impedance of the MSL 102 as shown in the following equation:Z=√{square root over (Z0×Z1)},where Z is the impedance of the λ/4 matching circuit 106, Z0 is the impedance of the MSL 102, and Z1 is the impedance of the input end of the antenna part 101.
With respect to impedance matching, Japanese Laid-Open Patent Application No. 6-021715 discloses a planar antenna having a triplate structure. In this planar antenna, a circular microstrip antenna (MSA) element having a hole in its center is employed as a radiation element, so that the input impedance of the radiation element is made variable by changing its ring ratio. Further, the shape and size of the end part of a feeder and the distance between the end part of the feeder and the center of the radiation element are made variable. As a result, impedance matching is achieved with a simple structure without reducing the antenna gain (radiation efficiency).
The above-described technique, however, has the following disadvantages.
The matching circuit is a resonance circuit, and has frequency components. Therefore, the matching circuit may affect the frequency characteristics of the antenna. For example, since the matching circuit allows matching only at a specific frequency, the frequency band of the antenna is narrowed.
Further, since an extension circuit up to the antenna input part is increased in length, the antenna is more likely to be affected by the electric characteristics of a dielectric, such as dielectric loss.
The antenna area may be increased as a method of increasing gain by changing the antenna pattern. However, considering interconnection line density, this method is not effective as means of increasing the gain of a rectangular antenna.